1. Field of the Invention
The invention relates generally to rolling cutter earth boring drill bits used for the exploration and retrieval of petroleum and other minerals from the earth. In particular, the invention is a new form of cutting insert that allows a higher percentage of wear resistant material on the gauge face of the cutter of a rolling cutter earth boring drill bit than previously had been practically possible.
2. Description of the Related Art
The practice of horizontal drilling has had a major impact on modern rolling cutter rock bits. Not only are the bearing systems in these bits challenged in horizontal drilling, the cutting structures, particularly the gauge cutting elements, are severely stressed.
In order to improve the useful life of the gauge cutting elements, the practice of placing multiple rows of gauge reaming inserts on the gauge faces of the rolling cutters has become commonplace. The inserts are usually arranged such that they utilize the maximum amount of the gauge face surface area as possible. To further improve wear resistance, the gauge reaming inserts are often supplied with the wear surfaces coated with diamond or other superhard materials. Some of these gauge surface insert designs are shown in U.S. Pat. Nos. 3,952,895, 5,145,016, 5,351,770, 5,353,885, 5,542,485, and 5,671,817.
A limitation in all these designs, however, is that the amount of wear resistant material exposed at the surface is limited by the round shape of the insert. Since these inserts are interferingly fitted into sockets in the rolling cutter, their placement is profoundly affected by the locations of the other inserts in the cutter. The placement limitations reduce the total amount of wear resistant material that may be placed on the gauge face portion of the rolling cutter.
One way to address the placement problem is to utilize non-circular inserts as shown in U.S. Pat. Nos. 5,421,423 and 5,890,550. The problem with non-circular inserts is that it is difficult and expensive to make non-circular holes in the rolling cutter that will properly retain the inserts. Although non-circular inserts are shown which are interferingly fitted into sockets in a rolling cutter, they are not in common use due to this difficulty.
The present invention is a wear resistant cutting insert for use in rolling cutter drill bits. The insert has a cylindrical shank that is readily fitted in an interfering manner into cylindrical sockets formed in the rolling cutter. The surface of the insert that engages the formation, however, is non-circular, and preferably, oval shaped. To prevent the insert from turning in the socket, a shallow counterbore is formed about the socket that accommodates the shape of the top, non-cylindrical portion of the insert. The depth of the recess is made slightly greater than the length of the shank of the insert so that the load applied to insert during operation is not transmitted through the shank to the bottom of the socket in the cutter.